where the dot denotes derivative with respect to time \(t\) , \(\rho \)  is the
total energy density in the Universe
and \(\varkappa = 0, \pm 1\)  is the parameter,
introduced in Section REF , that discriminates
the Euclidean 3-space
(\(\varkappa =0\) ) and curved 3-spaces.
The Friedmann equation is nothing but the \((00)\) -component
of the Einstein equations of general relativity,
\(R_{00} - \frac{1}{2}g_{00}R = 8\pi T_{00}\) ,
specified to the FLRW metric. Observationally,
the spatial curvature of the Universe is very small:
the last,
curvature term in the right-hand side of
Eq. (REF ) is small compared to
the energy density term [1]},
\(\frac{1/a^2}{8 \pi \rho /(3M_{\rm Pl}^2)} < 0.005 \; ,\) 
